According to the official announcement, forces are being contributed by Saudi Arabia, the United Arab Emirates, Egypt, Jordan, Bahrain, Sudan, Kuwait, Morocco, Pakistan, Tunisia, Oman, Qatar, Malaysia and several other nations. This exercise will reportedly last for 18 days, and during that time the airspace over northern Saudi Arabia will be closed to air traffic. This will be the largest military exercise in the history of the region, and it comes amid rumors that Saudi Arabia and Turkey are preparing for a massive ground invasion of Syria.
If you were going to gather forces for an invasion, this is precisely how you would do it. Governments never come out and publicly admit that forces are moving into position for an invasion ahead of time, so “military exercises” are a common excuse that gets used for this sort of thing.